Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.
Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also called photovoltaic (PV) devices, are a type of photosensitive optoelectronic device that is specifically used to generate electrical power. PV devices, which may generate electrical energy from light sources other than sunlight, can be used to drive power consuming loads to provide, for example, lighting, heating, or to power electronic circuitry or devices such as calculators, radios, computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that operation may continue when direct illumination from the sun or other light sources is not available, or to balance the power output of the PV device with a specific application's requirements. As used herein the term “resistive load” refers to any power consuming or storing circuit, device, equipment or system.
Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light.
Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector is used in conjunction with a current detecting circuit which measures the current generated when the photodetector is exposed to electromagnetic radiation and may have an applied bias voltage. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to electromagnetic radiation.
These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias. As a general rule, a photovoltaic cell provides power to a circuit, device or equipment, but does not provide a signal or current to control detection circuitry, or the output of information from the detection circuitry. In contrast, a photodetector or photoconductor provides a signal or current to control detection circuitry, or the output of information from the detection circuitry but does not provide power to the circuitry, device or equipment.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g., crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation to generate electric charge carriers.
PV devices may be characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications, and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. Present commercially available amorphous silicon cells have stabilized efficiencies between 4 and 8%.
PV devices may be optimized for maximum electrical power generation under standard illumination conditions (i.e., Standard Test Conditions which are 1000 W/m2, AM1.5 spectral illumination), for the maximum product of photocurrent times photovoltage. The power conversion efficiency of such a cell under standard illumination conditions depends on the following three parameters: (1) the current under zero bias, i.e., the short-circuit current ISC, in Amperes (2) the photovoltage under open circuit conditions, i.e., the open circuit voltage VOC, in Volts and (3) the fill factor, ff.
PV devices produce a photo-generated current when they are connected across a load and are irradiated by light. When irradiated under infinite load, a PV device generates its maximum possible voltage, V open-circuit, or VOC. When irradiated with its electrical contacts shorted, a PV device generates its maximum possible current, I short-circuit, or ISC. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the product of the current and voltage, I×V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, ISC×VOC. When the load value is optimized for maximum power extraction, the current and voltage have the values, Imax and Vmax, respectively.
A figure of merit for PV devices is the fill factor, ff, defined as:ff={ImaxVmax}/{ISCVOC}  (1)where ff is always less than 1, as ISC and VOC are never obtained simultaneously in actual use. Nonetheless, as ff approaches 1, the device has less series or internal resistance and thus delivers a greater percentage of the product of ISC and VOC to the load under optimal conditions. Where Pinc is the power incident on a device, the power efficiency of the device, ηP, may be calculated by:ηP=ff*(ISC*VOC)/Pinc 
To produce internally generated electric fields that occupy a substantial volume of the semiconductor, the usual method is to juxtapose two layers of material with appropriately selected conductive properties, especially with respect to their distribution of molecular quantum energy states. The interface of these two materials is called a photovoltaic junction. In traditional semiconductor theory, materials for forming PV junctions have been denoted as generally being of either n or p type. Here n-type denotes that the majority carrier type is the electron. This could be viewed as the material having many electrons in relatively free energy states. The p-type denotes that the majority carrier type is the hole. Such material has many holes in relatively free energy states. The type of the background, i.e., not photo-generated, majority carrier concentration depends primarily on unintentional doping by defects or impurities. The type and concentration of impurities determine the value of the Fermi energy, or level, within the gap between the conduction band minimum and valance band maximum energies. The Fermi energy characterizes the statistical occupation of molecular quantum energy states denoted by the value of energy for which the probability of occupation is equal to ½. A Fermi energy near the conduction band minimum energy indicates that electrons are the predominant carrier. A Fermi energy near the valence band maximum energy indicates that holes are the predominant carrier. Accordingly, the Fermi energy is a primary characterizing property of traditional semiconductors and the prototypical PV junction has traditionally been the p-n interface.
The term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. Rectification is associated normally with a built-in electric field which occurs at the junction between appropriately selected materials.
Conventional inorganic semiconductor PV cells employ a p-n junction to establish an internal field. High-efficiency PV devices are typically produced on expensive, single crystal growth substrates. These growth substrates may include single crystal wafers, which can be used for creating a perfect lattice and structural support for the expitaxial growth of active layers, also known as “epilayers.” These epilayers may be integrated into PV devices with their original growth substrates intact. Alternatively, those epilayers may be removed and recombined with a host substrate.
In some instances, it may be desirable to transfer the epilayers to host substrates that exhibit desirable optical, mechanical, or thermal properties. For example, Gallium Arsenide (GaAs) epilayers may be grown on Silicon (Si) substrates. However, the electronic quality of the resulting material may be insufficient for certain electronic applications. Therefore, it may be desirable to preserve the high material quality of the lattice-matched epilayers, while allowing the integration of those epilayers into other substrates. This may be accomplished by a method known as epitaxial liftoff. In epitaxial liftoff processes, epilayers may be “lifted off” growth layers and recombined (e.g., bonded or adhered) to a new host substrate.
Although they may provide desirable epitaxial growth characteristics, typical growth substrates can be thick and create excess weight, and the resulting devices tend to be fragile and require bulky support systems. Epitaxial liftoff may be a desirable way to transfer epilayers from their growth substrates to more efficient, light-weight, and flexible host substrates. Given the relative scarcity of typical growth substrates and the desirable characteristics that they impart on resulting cell structures, it may be desirable to recycle and/or reuse growth substrates in subsequent epitaxial growths. However, prior attempts to reuse growth wafers have either resulted in reduced efficiency or required abrasive “polishing” of the wafer by removing the top several micrometers of material from the wafer.
U.S. Patent Publication No, 2010/0047959 describes a method for selectively freeing an epitaxial layer from a single crystal substrate. The described method includes the deposition of a first buffer layer, an etch stop layer, a second buffer layer, and a separation layer. Above the separation layer, a sequence of semiconductor layers is deposited to form a cell. The method then comprises etching the separation layer, whereby semiconductor layers are pulled away from substrate and the associated buffer layers and etch stop layer. However, the specification does not describe a method of preparing the liberated substrate for reuse, such as, for example, selectively etching away the buffer layers and/or etch stop layers. Accordingly, there remains a need to develop methods that preserve the integrity of growth substrates for reuse.